Equilibrium constant in terms of fugacity

These quantities are not in general dimensionless. One can define in an analogous way an equilibrium constant in terms of fugacity Kf, etc. At low pressures Kp is approximately related to K by the equation K Kp/(p )lv, and similarly in dilute solutions Kc is approximately related to K by K Kc/(c )lv however, the exact relations involve fugacity coefficients or activity coefficients [24]. 

The form taken by the equilibrium constant depends on the type of expression which is substituted in the above equation for the purpose of expressing the chemical potentials in terms of the composition this in its turn dex>ends on additional physical knowledge concerning whether or not the real system in question may be approximately represented by means of a model, such as the perfect gas or the ideal solution. If the system does not approximate to either of these models it is still possible, of course, to formulate an equilibrium constant in terms of fugacities or in terms of mole fractions and activity coefficients. However, this isapurely formal process the fugacities and activity coefficients are themselves defined in terms of the chemical potentials and therefore the knowledge contained in equation (10 1) is in no way increased, but is obtained in a more convenient form. 

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